Magnified telescopic sights, such as rifle scopes and spotting scopes, for example, frequently suffer from very small exit pupil size. When the exit pupil diameter of the device is approximately the same size or smaller than the diameter of the pupil of the operator's eye, it is increasingly difficult to keep the eye within the effective eyebox of device. Moving the eye out of the eyebox results in severe vignetting or total loss of the observed image.
For example, if the diameter of the exit pupil of a telescope is about 2 millimeters (mm) or less, the operator must try to keep his/her head extremely stable, even if the device is equipped with a tripod, in order to keep the eye within the eyebox. Very small, sub-millimeter movements of the operator's head can have significant impact. Therefore, even people who are trained to work with instruments having tight eyeboxes, such as snipers or competitive sports shooters, for example, may be not able to keep their heads sufficiently stable, particularly under such circumstances as intensive physical effort (for example, after a fast run), stress, or exhaustion. In addition, it may take several seconds to achieve proper match of the operator's eye and the eyebox of the device.
In view of these difficulties, it is highly desirable to have the size of the eyebox significantly larger than the size of the pupil of the eye. However, conventionally this requirement leads to larger front optics and increased total length of the device, particularly for devices with higher magnification. First principles of geometric optics dictate that it is not possible to generate a large exit pupil in combination with a relatively small entrance pupil and magnification of the telescope significantly larger than one. This is because the magnification of an afocal telescope is given by the ratio of the diameter of the entrance pupil to the diameter of the exit pupil. Thus, for example, if the diameter of exit pupil is 2 mm and the required magnification is 25×, the resulting diameter of entrance pupil is 50 mm. A typical length estimate for such a telescope is then at least 11-12 inches, if not significantly more. Doubling the diameter of the exit pupil to 4 mm, which would significantly improve matching the eye to the eyebox, causes a corresponding increase in the entrance pupil diameter to 100 mm, and this results in a telescope that is too heavy and too long to use as a riflescope or portable spotting telescope.